1. Field of the Invention
This invention relates in general to an integrated optical device and method for manufacturing same, and in particular to an integrated optical device having multiple nanometer-scale level structures and method for manufacturing same.
2. Description of the Related Art
Conventional spectrometers typically use dispersive elements to separate light into its spectral components, requiring space and precise alignment of delivery optics. The requirement of holding imaging optics and dispersive optics in precise alignments adds substantially to the size, weight, and assembly costs of optical systems. The size, weight and cost of spectrometers can be reduced by building an integrated optical device on the image sensor. Such an integrated system has further advantages relative to a grating spectrometer in its robust alignment and its sensitivity.
Several types of integrated spectrometers have been demonstrated by using micro-scale Fabry-Perot etalons. A Fabry-Perot etalon is typically made of a transparent medium bounded by two reflecting surfaces to create an optical cavity. The transmission spectrum of the cavity exhibits peaks of transmission corresponding to resonances of the optical cavity. The position of resonances depends very sensitively on the cavity length and the index of the material in the cavity. Required tolerances for cavity length can be of nanometer order, making fabrication challenging.
The standard way to micro-fabricate optically flat steps with nanometer-scale height control is to sequentially etch levels into a dielectric material. Each level is done in a single lithographic and subsequent etching step. There are ways to reduce the number of process steps, such as “combinatorial etching.” Essentially, one needs to perform a square-root of micro-fabrication steps for the overall step amount. Even this approach leaves ˜31 micro-fabrication steps for a structure with 1,000 levels. Because each micro-fabrication step is time consuming and adds costs, minimizing the amount of fabrication steps is desired. For comparison, a typical commercial chip, depending on its complexity, requires 8-32 lithography steps. The resulting fabrication of a 1,000 level structure by digital etching is quite an effort. Other approaches use grey-scale lithography to fabricate steps in a single lithograph step, but the variability of etch processes and material homogeneity usually limit the number of truly distinct levels to around 100-400 levels.